1. Field of the Invention
The present invention relates to a ceramic matrix composite obtained by combining ceramic fibers having a coat layer such as a sliding coat layer or a protective reaction barrier layer with a matrix containing SiC as a main component and to a method of manufacturing the ceramic matrix composite and, more particularly, to a ceramic matrix composite in which SiC of a matrix is formed by reaction sintering, fibers and/or a coat layer is suppressed from being eluted in the matrix during the reaction sintering to prevent a sliding function from being degraded and to increase breakdown energy, thereby designing a sound composite system, and to a method of manufacturing the ceramic matrix composite.
2. Description of the Related Art
In general, a ceramic sintered body has strength which is less degraded up to a high temperature, and is more excellent than a conventional metal material in hardness, electric insulating properties, abrasion resistance, corrosion resistance, light-weight properties, and the like. For this reason, the ceramic sintered body is popularly used in a large field as an electronic material or a structure material such as a heavy electric equipment part, an aircraft part, an automobile part, an electronic equipment, a precision instrument, or a semiconductor device material.
However, the ceramic sintered body is easily affected by tensile stress rather than compression stress, and, especially, the ceramic sintered body is broken by the tensile stress at once. That is, the ceramic sintered body disadvantageously has so-called high brittleness. For this reason, in order to make it possible to apply a ceramic part to a portion whose reliability should be high, high toughness of the ceramic sintered body or an increase in breakdown energy are strongly demanded.
More specifically, a ceramic structure part used as a gas turbine part, an aircraft part, an automobile part, or the like requires high heat resistance, high refractory, and high reliability. In order to reply to this demand, studies for realizing the following ceramic composite material have advanced in domestic and foreign laboratories. That is, the ceramic composite material is improved on a breakdown toughness value, a breakdown energy value, or heat resistance, and impact strength in such a manner that composite elements such as a whisker, a blade, and particles are dispersed in a matrix sintered body to be combined with each other.
As such a ceramic composite material which is excellent in refractory characteristics, a ceramic matrix composite obtained by combining ceramic fibers with a ceramic matrix containing silicon carbide (SiC) as a main component especially attracts attention. In particular, a ceramic matrix composite obtained by combining SiC fibers with an SiC matrix attracts attention with respect to its general-purpose properties. A large number of SiC continuous fibers are gathered with each other to form yarns, and the yarns are two-dimensionally or three-dimensionally woven to form a fabric. The yarns or fabric can be applied to various shaped parts. However, in this composite ceramic material, the matrix and the continuous fibers consist of the same material, i.e., SiC. For this reason, the matrix and the continuous fibers may be disadvantageously separated on the same plane.
More specifically, in the ceramic matrix composite, it is very important to properly control the interfacial crystal force between the matrix and the fibers. If the interface state is not proper, the fibers are brought into tight contact with the ceramic, composite effects such as pullout or bridging cannot be performed, breakdown caused by brittleness easily occurs.
In recent years, as a countermeasure against the above problem, it is known that a sliding coat layer is effectively formed on the surface of fibers to exhibit sliding between the fibers and a matrix. More specifically, the following technique has been developed. That is, a coat layer consisting of a material other than that of SiC fibers is formed on the SiC fibers and used as a sliding coat layer, so that the fibers are easily pulled out of the matrix during growing of cracks to improve breakdown energy. As a coat layer suitable for the SiC fibers, boron nitride (BN) may be used. The present inventor has examined various types of sliding coat layers.
However, even if only such a sliding coat layer is formed, when a matrix is formed by reaction sintering, the sliding coat layer is disadvantageously deteriorated or eliminated. In particular, a reaction sintering method for causing a preform consisting of ceramic fibers to be impregnated with molten Si, performing reaction sintering between the molten Si and C powder with which the preform is impregnated, and forming a matrix containing SiC as a main component is applied, the reactive molten Si may react with the BN layer to elute B generated by decomposing the BN layer into the matrix, thereby eliminating the BN layer, and the SiC fibers themselves may be eluted.
In contrast to this, the following conventional technique is also be proposed. That is, a sliding coat layer of a fiber surface is further coated to form a barrier layer serving as a protective reaction layer for suppressing reaction between a matrix and the fibers. However, even if this barrier layer is formed, it is difficult to completely prevent the fibers from being eluted in the matrix material.
On the other hand, as the method of forming an SiC matrix, a CVI method, a precursor method, a powder sintering method, or the like is available. A reaction sintering method in which a melted metal is impregnated into a fiber preform to perform reaction sintering attracts attentions as a method which is suitable for mass-production. In this reaction sintering method, a carbon (C) powder is arranged into gaps between the fibers of the preform using SiC fibers, and a molten silicon (Si) is impregnated into the resultant compact to perform a reaction, thereby forming an SiC matrix. According to the reaction sintering method, a continuous fiber composite ceramic can be manufactured in relatively easy steps, and the product has excellent mechanical characteristics at a high temperature. In reaction sintering, the matrix rarely contracts, so that the matrix can be easily combined with the continuous fibers.
For this reason, a ceramic matrix composite which employs BN-coated SiC fibers as a composite material and employs reaction sintered SiC as a matrix is considerably expected.
However, when a ceramic matrix composite containing SiC as a matrix is manufactured by reaction sintering using BN-coated SiC fibers as a composite material, a sufficient sliding effect cannot be always obtained, and the matrix and fibers are often separated from each other on the same plane during growing of cracks. The separation on the same plane is conspicuous when the BN coating is thin, and the separation on the same plane tends to decrease as the coating thickness is increased. However, when the thickness of the BN coating is increased, an increase in cost is caused. The increase in cost prevents the ceramic matrix composite from being realized.